Sat, Jul 04, 2009
 At Florida's Kennedy Space Center, technicians dressed in "bunny suits," or clean-room attire, prepare the Dawn spacecraft for next Wednesday's launch. Dawn is scheduled to visit two asteroids in the coming years. Courtesy of NASA
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Tucson, Arizona | Published: 09.19.2007
Don't hold your breath, Pluto fans, but a NASA spacecraft leaving for the asteroid belt next week could make a case for classifying planets based on geology rather than size and influence, according to a Tucson scientist working on the Dawn mission.
The National Aeronautics and Space Administration's Dawn mission, scheduled to lift off from Florida's Cape Canaveral Air Force Station next Wednesday, will visit the asteroids Ceres and Vesta.
Vesta is a dry rock 360 miles around at its equator, and Ceres is possibly a water-bearing object roughly 590 miles around at its equator. Ceres was the first asteroid to be discovered (in 1801), and it's the largest, most massive object in the asteroid belt. It recently was bumped up to dwarf-planet status.
Pluto, the ninth planet in our solar system since it was discovered at Flagstaff's Lowell Observatory in 1930, was busted down to dwarf-planet status last year in a controversial move by the International Astronomical Union.
Exploration of Ceres and Vesta could reveal much about the dramatically different bodies — not totally unlike the differences between some planets in our solar system, said Mark V. Sykes, director of the Tucson-based Planetary Science Institute. Sykes is a co-investigator and science team member on the Dawn mission.
"I've been arguing for a geophysical perspective," Sykes said about the classification of solar system objects. Ceres "is not like any other asteroid."
The dwarf planet may have water, and there was one recent observation — though not yet repeated — that it may even have a thin atmosphere.
"It may have a subsurface ocean," Sykes said cautiously. "And where there's water, it always raises the question: Could there be life?"
Vesta, though it's thought to be less Earthlike, also is interesting. Sykes said a massive impact crater near its south pole may be deep enough to give Dawn's instruments a peek at Vesta's interior.
The Planetary Science Institute has considerable involvement in Dawn, a twice-canceled mission that Sykes said is probably stronger now than before due to its brushes with oblivion.
Dawn's Gamma Ray and Neutron Detector device was designed, in part, by the institute's Bill Feldman. The instrument, which will penetrate to a depth of about 3 feet, will help analyze the bodies' composition.
And simulation software written by the Planetary Science Institute's Pasquale Tricarico will help predict what Dawn may encounter while skimming Vesta at less than 40 miles of altitude.
Tricarico said there is particular interest in what will happen while Dawn passes over Vesta's huge crater. He said the crater may cause gravitational variations that will "kick" Dawn as it passes overhead, possibly putting it in a dangerously low elliptical orbit.
There is plenty of time to look into these questions; Dawn isn't scheduled to drop by Vesta until September 2011, and Ceres in February 2015.
It's a slow ride, but Sykes said the mission hardware itself is of interest for that very reason. Dawn is the first U.S. science mission to use ion propulsion, a slow-but-steady approach to getting from Point A (Earth) to anywhere.
A conventional Delta 2 rocket — the same workhorse that sent NASA's Phoenix Mars spacecraft on its way to the red planet last month — will push Dawn beyond Earth's clutches. But then the ion drive will take over, running for years to push Dawn toward the asteroids.
NASA compares Dawn's ion propulsion power to a car that takes four days to go from zero to 60 mph, or the amount of power one would use to hold a piece of notebook paper in his hand.
On the other hand, it will get where it's going on solar power and a mere 937 pounds of xenon gas.
Sykes said the mission to two objects would have been impossible without ion propulsion; an affordable rocket couldn't have taken off with enough conventional fuel to accomplish both encounters.
As for the flap over whether Pluto is a first-class planet or a dwarf planet, Sykes said his definition of a planet is something you could teach elementary school kids: "Planets are round things that orbit stars. Period."
View photos of the Dawn spacecraft at azstarnet.com/slideshows
● Contact reporter Dan Sorenson at 573-4185 or dsorenson@azstarnet.com.
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